CAPILLARY ACTION. 



563 



The unit in which a is expressed is 1cm. xlO~"; a is therefore the twenty- 

 millionth part of a centimetre for mercury, the thirty-millionth for water, and 

 the forty-millionth part for alcohol. Quincke, however, found by direct experi- 

 ment that certain molecular actions were sensible at a distance of a two- 

 hundred-thousandth part of a centimetre, so that we cannot regard any of these 

 numbers as accurate. 



ON SURFACE-TENSION. 



Definition. The tension of a liquid surface across any line drawn on the 

 surface is normal to the line, and is the same for all directions of the line, 

 and is measured by the force across an element of the line divided by the 

 length of that element. 



Experimental Laws of Surface-tension. 



1. For any given liquid surface, as the Surface which separates water from 

 air, or oil from water, the surface-tension is the same at every point of the 

 surface and in every direction. It is also practically independent of the curva- 

 ture of the surface, although it appears from the mathematical theory that 

 there is a slight increase of tension where the mean curvature of the surface 

 is concave, and a slight diminution where it is convex. The amount of this 

 increase and diminution is too small to be directly measured, though it has 

 a certain theoretical importance in the explanation of the equilibrium of the 

 superficial layer of the liquid where it is inclined to the horizon. 



2. The surface-tension diminishes as the temperature rises, and when the 

 temperature reaches that of the critical point at which the distinction between 

 the liquid and its vapour ceases, it has been observed by Andrews that the 



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